Heat exchanger units tend to be large and therefore costly in the service of gas heating or cooling and evaporation and/or condensation of volatile liquids in the presence of a non-condensible gas. The problem with the heat exchange units is poor heat and mass transfer coefficients in the interaction with the heat exchange surfaces. It is known that heat transfer coefficients for gases, as well as heat and mass transfer coefficients for evaporation and condensation in the presence of a non-condensible gas, are low. The low transfer coefficients are due to the difficulty of the conduction of heat and the diffusion of particle components through the stagnant gas boundary layer associated with the transfer surfaces. As an example, addition of a non-condensable gas to a condenser has been shown to reduce drastically heat transfer coefficients as reported by D. F. Othmer, "The Condensation of Steam", I+EC, 21, 1929, No. 6, pages 576-583 describing the results of a series of experiments involving known amounts of air in a steam condenser. The results suggest that a film of non-condensable gas and vapor collects about the condensate film requiring that steam diffuse through the stagnant layer of air before condensing on the surface. Heretofore, the methods of improving the heat transfer rate for a gas or for evaporation and condensation in the presence of a non-condensable gas have involved enlarging the transfer area with fins and other shapes or by increasing transfer coefficients by greater gas phase velocity, thereby causing gas phase pressure drop. Both methods are costly solutions to a difficult problem.
A need has therefore been felt for increasing the heat transfer rate for a gas and for evaporation and condensation in the presence of a non-condensible gas at a transfer surface.
It is therefore a feature of the present invention to provide apparatus and technique for enhancing the heat transfer coefficents between a gas and heat transferring surface.